Inorganic photoconductive materials recently in use include amorphous silicon, amorphous selenium, cadmium sulfide, zinc oxide, and the like. However, some of these materials are expensive because of difficulties in production thereof, while others are toxic and disadvantageous from the standpoint of environmental protection.
On the other hand, as organic photoconductors, ones of the type comprising, in particular, a charge-generating material and a charge-transporting material which respectively perform their functions are proposed extensively (e.g., U.S. Pat. No. 3,791,826). In this type, there is the possibility that a high-sensitivity electrophotographic photoreceptor might be obtained by using a substance which efficiently generates carriers (The term "carriers" means "charges"; the same applies hereinafter) as the charge-generating material in combination with a substance having high charge-transporting ability as the charge-transporting material.
Of these materials, the charge-transporting material is required to efficiently receive the carriers generated in the charge-generating material upon light irradiation in an electric field and permit them to rapidly move through the photosensitive layer to extinguish the surface charges promptly. The speed at which carriers move per unit electric field is called carrier mobility. A high carrier mobility means that carriers rapidly move in the charge-transporting layer. Any charge-transporting material has its intrinsic carrier mobility and, hence, it is necessary that for attaining a high carrier mobility, a material having a high carrier mobility be employed. However, the attainable carrier mobilities have not yet reached a sufficient level.
Further, in the case of applying a charge-transporting material after dissolving it in an organic solvent along with a binder polymer, it is necessary to form a thin homogeneous organic coating film free from crystallization and pinhole formation. This is because when a high electric field is applied to the thin film obtained, the part having microcrystals or pinholes undergoes dielectric breakdown or causes noise.
In addition to the satisfactory properties of the charge-generating material and of the charge-transporting material, it is also important that carriers should be efficiently injected from the charge-generating material into the charge-transporting material, i.e., from the charge-generating layer into the charge-transporting layer. This injection of charges depends on the properties of the interface between the charge-generating material (or charge-generating layer) and the charge-transporting material (or charge-transporting layer) and varies with combinations of various materials. Since a charge-transporting material should meet various requirements as described above, charge-transporting materials having a variety of properties are being developed.
Among conventional charge-transporting materials, the styryl compound represented by formula (A): ##STR6## is, for example, proposed in JP-A-60-174749. (The term "JP-A" as used herein means an "unexamined published Japanese patent application.")
Moreover, a styryl compound represented by formula (B): ##STR7## (wherein R.sup.1 represents an optionally substituted alkyl group or an aryl group which may have a substituent group, R.sup.2 represents a hydrogen atom, an optionally substituted alkyl group, or an aryl group which may have a substituent group, and Ar represents an aryl group which may have a substituent group) is proposed in JP-A-60-175052.
Furthermore, compounds similar to the compound (B) described above are proposed in, for example, JP-A-62-120346, JP-A-1-217357, JP-A-4-57056, and JP-A-4-292663.
On the other hand, a styryl compound represented by formula (C): ##STR8## (wherein R.sup.1, R.sup.3, and R.sup.5 each represents a hydrogen atom, an alkyl group, an aryl group which may have a substituent group, an optionally substituted aralkyl group, or an optionally substituted heterocyclic group; R.sup.2, R.sup.4, and R.sup.6 each represents an aryl group which may have a substituent group, an optionally substituted aralkyl group, or an optionally substituted heterocyclic group; R.sup.1 and R.sup.2, R.sup.3 and R.sup.4, and R.sup.5 and R.sup.6 may be bonded to each other to form a ring; and R.sup.7, R.sup.8, and R.sup.9 each represents a hydrogen atom, an alkyl group, an alkoxy group, an aralkyl group, or an aryl group) is proposed in JP-B-6-93124. (The term "JP-B" as used herein means an "examined Japanese patent publication.")
Compounds similar to the compound (C) described above are proposed in JP-A-63-163361 and JP-A-6-332206. In JP-A-4-292663 is proposed a hydrazone compound represented by formula (D): ##STR9## (wherein R.sup.1, R.sup.2, R.sup.3, R.sup.4, R.sup.5, and R.sup.6 may be the same or different and each represents a hydrogen atom, an optionally substituted alkyl group, an optionally substituted alkoxy group, an optionally substituted aralkyl group, or an aryl group which may have a substituent group; Ar.sup.1, Ar.sup.2, Ar.sup.3, Ar.sup.4, Ar.sup.5, and Ar.sup.6 may be the same or different and each represents a hydrogen atom, an optionally substituted alkyl group, an aryl group which may have a substituent group, an optionally substituted aralkyl group, or an optionally substituted heterocyclic group; and l, m, and n each represents 0 or 1; provided that Ar.sup.1, Ar.sup.2, Ar.sup.3, Ar.sup.4, Ar.sup.5, and Ar.sup.6 should not be a hydrogen atom at the same time).
The demand for charge-transporting materials is growing more and more, with which there is a desire for a newer material which is capable of satisfying various requirements.
In JP-A-4-57056, for example, there is a description to the effect that the compound (E) specified below partly separated out as crystals during the preparation of a photoreceptor because of the poor solubility of the compound. ##STR10##
Furthermore, in JP-A-6-332206, there is a description to the effect that a charge-transporting layer comprising the compound (F) specified below and a binder polymer develops cracks. ##STR11##
Further, JP-A-2-226159 discloses a compound represented by the following general formula G: ##STR12## wherein X represents an aryl, arylene, heterocyclic, diarylamino or triarylamino group which may have a substituent group; Y represents a group selected from the group consisting of: ##STR13## or a single bond; n represents an integer of 0 or 1; m represents an integer of from 1 to 3; R.sub.1 and R.sub.2 may be the same or different and each represents an alkyl, aryl, aralkyl, cyano or nitro group which may have a substituent group or a halogen atom; and R.sub.3 represents an alkyl, aryl or aralkyl group which may have a substituent group. However, this is a complex compound having two phenyl groups which are connected to each other via Y to form another ring.
The preparation of the foregoing various triphenylamine derivatives to be used as known charge-transporting materials can be accomplished by a process which comprises reacting the corresponding compound having a triphenylamine skeleton with a Vilsmeier reagent prepared from a halogenating reagent such as phosphorus oxychloride and a formylating agent such as N,N-dimethylformamide to obtain an iminium intermediate, and then hydrolyzing the iminium intermediate with an alkaline aqueous solution to prepare a formyl-substituted triphenylamine derivative which is then reacted with a predetermined phosphorous acid ester, as described in "Jikken Kagaku Koza", vol. 14, page 688 (published by Maruzen).
However, if the foregoing process causes the reaction of triphenylamine with one equivalent of a Vilsmeier reagent, the resulting monoiminium salt has an extremely deteriorated nucleophilicity that makes itself difficult to react with another Vilsmeier reagent, making it impossible to efficiently produce the desired diiminium salt. Accordingly, the foregoing process can easily accomplish the synthesis of a monoaldehyde of triphenylamine but can hardly accomplish the synthesis of a diformylation product of triphenylamine. Even if the foregoing reaction is effected in the presence of a large amount of a Vilsmeier reagent or over an extended period of time, the resulting yield is very low. For example, JP-A-7-173112 discloses that the yield of 4,4'-diformyltriphenylamine is 39.5% and the yield of 4-methyl-4,4'-diformyltriphenylamine is only 11.8%.
Bouanane et al. reported that ordinary Vilsmeier reaction of triphenylamine produces 4,4'-diformyltriphenylamine in a yield of 75% (C. R. Hebd. Seances Acad. Sci., Ser. C, Vol. 279, No. 5, pp. 187-190). However, according to the inventors' experiment, 4,4'-diformyltriphenylamine cannot be obtained in such a high yield. Further, this reaction process is very disadvantageous for the reaction of the intermediate with a further Vilsmeier reagent that produces a triiminium salt, making it very difficult to synthesize a triformylation product of triphenylamine.
As mentioned above, no processes for efficient synthesis of 4,4'-diformultriphenylamine derivative and 4,4',4"-triformyltriphenylamine derivative have been known.
Accordingly, an object of the present invention is to provide a novel charge-transporting material which gives a stable film attaining a high carrier mobility and which, when used in an electrophotographic photoreceptor, is also excellent in various properties.
It is another object of the present invention to provide an electrophotographic photoreceptor comprising a novel material having the foregoing properties as a charge-transporting material.
It is a further object of the present invention to provide a process for the efficient preparation of a novel charge-transporting material having the foregoing properties.
It is a still further object of the present invention to provide an improved process for the preparation of an intermediate substance useful for the preparation of the novel charge-transporting material.